Airflow-measurement technologies

Tue, 2008-04-01 12:00

I must state our objections to the December Design Solutions article “Pitot-Tube Technology Helps Maintain Proper Airflow in Fitness Facility,” the intent of which appears to have been to cause irreparable and material harm to advanced thermal-dispersion (TD) airflow-measurement technology.

Advanced TD airflow-measurement technology (not hot-wire anemometry) is used in laboratory, health-care, and educational facilities to ensure healthy indoor-air quality and economy of operation. TD devices are the only duct-averaging instruments factory-calibrated to National Institute of Standards and Technology- (NIST-) traceable reference standards and the only ones using a unique combination of highly stable components with microprocessor-based and totally independent sensing elements in a cross-sectional array. Their performance potential can be seen at NIST's Advanced Measurement Laboratory in Gaithersburg, Md., where more than 350 devices have functioned without a single reported problem for five or six years.

Allow me to enumerate some of the problems we have with the article:

The terms “Pitot tube” and “hot wire” are used incorrectly in relation to the products described in the article, leading the reader to believe the products possess the same properties, capabilities, and limitations.

A Pitot-static tube is nothing like an “averaging” Pitot array. The two share only the sensing of velocity, or differential, pressure in determining velocity and calculating volume. The laboratory Pitot-static tube is a “primary” instrument with physical properties scientifically proven to provide a predictable level of measurement performance, albeit with known application limitations. The Pitot, or self-averaging, array is a bifurcated or dual-manifold device that “equalizes” a pressure within its length. A differential pressure is measured, assumed to be the “average” from numerous ports, and then made linear. The result is output to a host controller. Manufacturers claim that this technology “averages” the pressures in a duct cross-section. The use of a normal single-point velocity-pressure-to-velocity calculation requires the user to assume that averaging velocity pressure before velocity is determined at multiple points makes no difference to the result. Mathematically, a significant error is introduced.

Hot-wire anemometry is far removed from microprocessor-based TD technology. Their only similarity is the use of resistance devices and fluid flow in determining fluid velocity. Hot-wire devices have limitations TD sensors do not.

Moisture and debris in the air stream of a return fan will decrease the performance potential and increase the maintenance needs of any instrument. Pitot arrays, like any device that physically samples particulate-laden air, clog and must be cleaned, or their ability to detect variations in velocity profiles will be overcome. Because of the humid, unfiltered return air of its fitness center and pool, the Rochester Institute of Technology (RIT) should have installed duct-mounted sensors in the return-air plenum.

With intentional depressurization of a space, more-precise control is required to avoid undesirable effects. Condensation and infiltration of unconditioned air are symptoms of insufficient makeup air or insufficient control; they are not entirely the result of the degraded performance of instrumentation.

The article states that a comparison of the two technologies was made. The article includes a photograph that appears to depict a comparison test. The equipment in the photo is not the equipment installed in the RIT fitness center and discussed in the article.

The sensors in the photo are mounted in different intake planes and, therefore, have different volumetric profiles. The inner sensor is subject to a much larger airflow volume with a greater potential for particulate buildup than the sensor mounted farther away.

On close inspection, the photograph shows what appears to be particulate buildup on both sensors. Particulate buildup will affect any sensing instrument's operation and accuracy. In the case of Pitot arrays, because of very small sampling orifices, particulate buildup and obstructions will result in false readings and, at some point, failure to function. Yet, according to the RIT's manager of HVAC-systems support, the performance of the “Pitot-tube device didn't degrade at all, and it required no cleaning.”

A large relative area of through flow and a small surface area make TD sensing elements inherently immune to fouling from most common types of dirt plaguing air-sampling sensors. The binding effects of moisture, combined with the insulating properties of cotton materials, can degrade the thermal conductivity of individual thermal sensors, which require only a light cleaning for their performance potential to be restored.

The article does not indicate the time allowed for debris buildup during the comparison test and does not say whether the two devices were in equal states of cleanliness when the test was initiated.

The article states that the accuracy of a basic Pitot tube typically is 2 percent (of what is not indicated). This claim is unqualified, leading readers to believe that such a level of measurement performance is achievable in the field, regardless of the application and despite the uncertainty added by all of the other components needed in a velocity-pressure system to produce a linear velocity signal to a controller. Under ideal laboratory conditions, a Pitot array can produce measurement uncertainty of ±2 percent from a reference, but it cannot provide a combined total operational uncertainty to that degree.

Because fan-inlet conditions vary greatly and are unpredictable, the fan inlet is the least preferred place to measure velocity. However, if measurement technology is repeatable, some complementary instruments can be set up in the field to track similar fans (e.g., supply and return fans). With fan-inlet applications, accessibility for installation and maintenance usually is diminished, particularly with dual-inlet fans and many “space-efficient” air handlers.Len Damiano Ebtron Inc. Loris, S.C.